It is known to the inventor to generate a broad spectrum noise signal, or "white" noise, with a circuit that employs an avalanche breakdown of a pn junction. Various embodiments of this technique include the use of a Zener diode, the use of a lateral diode biased to "soft" breakdown, or the use of an emitter-collector breakdown of a bipolar junction transistor (BJT) that is operated with an open base circuit.
However, the noise produced by any of these three embodiments is characteristically of relatively low power. Also, the Zener diode embodiment typically requires a supply voltage in excess of the five volts that is readily available in many systems. The use of a voltage doubler, to increase the supply voltage for the Zener diode, typically contaminates the noise signal and requires a large amount of capacitance. The use of a voltage doubler also adds complexity and consumes substrate surface area, an important consideration when integrating the noise source onto an integrated circuit. The lateral diode embodiment exhibits a large and poorly controlled output impedance that tends to reduce the bandwidth of the noise source. For the BJT approach (also referred to as a BVceo approach) the breakdown voltage is often too near the minimum supply voltage, and is also too strong a function of the transistor current gain, to properly control. The output noise also typically exhibits a narrow bandwidth.
It is thus an object of the invention to provide a flat white noise spectrum that is generated with a supply voltage that is less than that required for pn junction breakdown-based noise sources, allowing operation with a single five volt supply, or with a lower voltage battery.
It is a further object of the invention to provide a white noise source having a lower output impedance, a higher noise power, and a wider noise bandwidth, than is obtained with a conventional junction breakdown white noise source.